In the field of graphic arts, it is well known to utilize as an imaging master a silver halide photographic element comprising a dimensionally-stable polyester film support. Polyester films are utilized in such elements because their dimensional stability characteristics are unsurpassed. However, because of the difficulty of achieving strong bonding of overlying hydrophilic colloid layers to such films, it is usually necessary to employ a latex subbing layer between a polyester film support and the overlying photographic layer, such as a silver halide emulsion layer or a backing layer. Latex subbing layers used to promote the adhesion of coating compositions to polyester film supports are very well known in the photographic art. Useful compositions for this purpose include interpolymers of vinylidene chloride such as vinylidene chloride/acrylonitrile/acrylic acid terpolymers or vinylidene chloride/methyl acrylate/itaconic acid terpolymers. Such compositions are described in numerous patents such as, for example, U.S. Pat. Nos. 2,627,088, 2,698,235, 2,698,240, 2,943,937, 3,143,421, 3,201,249, 3,271,178, 3,443,950 and 3,501,301. The latex subbing layer is typically overcoated with a second subbing layer comprised of gelatin which is typically referred to in the art as a "gel sub." Functional layers, such as silver halide emulsion layers containing gelatin or other hydrophilic colloid as a binder, are then applied over the gel sub layer.
Since photographic elements employed as imaging masters in the field of graphic arts typically have functional layers on both sides thereof, the polyester films used as supports are commonly provided on both sides with both a latex subbing layer and a gel sub layer. Thus, a typical photographic element of this type has a latex subbing layer, a gel sub layer and a silver halide emulsion layer on one side and has a latex subbing layer, a gel sub layer and a backing layer on the opposite side.
In photographic elements comprising a silver halide emulsion layer on one side and a backing layer on the opposite side, the backing layer has to fulfill many requirements and meeting all of these requirements simultaneously has proven to be extraordinarily difficult. Exemplary of the essential requirements for a backing layer adapted for use in a graphic arts imaging master are the following:
(1) The backing layer must function to prevent excessive curl. This is typically done by designing the backing layer to balance the expansion and contraction forces in the layers on the opposite side of the film support. When forces on both sides of the film support are equal, curl is essentially eliminated. However, a serious problem exists in using this technique in that providing such a backing layer adds to the forces which cause the photographic element to expand and contract with changes in humidity, thereby degrading dimensional stability.
(2) The backing layer must function to provide effective halation protection without leaving residual dye stain. Incorporating filter dyes in the backing layer can serve to provide protection against image degradation through light reflecting from the base to the air interface, a phenomenon known as "halation." When the photographic element is of the type which is handled in roomlight or under bright safelights, the filter dyes in the backing also act to prevent unwanted exposure. It is critically important that such dyes are removed in the processing steps so that there is no residual dye stain after processing. This requirement severely limits the choice of binders that can be used in the backing layer, since a backing layer with poor water-permeability characteristics will inhibit the dissolution and removal of the dyes during processing.
(3) The backing layer must function to absorb water-based inks used to correct image defects in a process known in the graphic arts as "opaquing." In certain graphic arts operations, the need to guarantee against image defects is very high. An example would be where the photographic element is used to create an etched image on a gravure printing cylinder. Under these conditions, correcting an image defect in the printing cylinder is most difficult and costly. The user will tend to liberal use of opaquing ink and the preferred surface for opaquing is the back side of the element since if it becomes necessary to remove the opaque, the image will not be damaged. Thus, it is highly desirable that a backing layer be receptive to water-based inks.
(4) The backing layer must function to promote effective vacuum contacting in a vacuum drawdown process. As described in Nitschke et al, U.S. Pat. No. 4,997,735, issued Mar. 5, 1991, silver halide photographic elements utilized as imaging masters in the graphic arts are commonly employed in contact exposure processes which involve juxtaposing two photographic elements and drawing a vacuum between them, usually in a device known in the art as a vacuum frame. To promote effective vacuum contacting, the backing layer of the imaging master must have an appropriate surface roughness and this is usually achieved by incorporating matting agents in the backing layer. As pointed out in the '735 patent, the use of a high concentration of gelatin as the binder in the backing layer makes it difficult to achieve the desired vacuum drawdown efficiency. To achieve such efficiency, the '735 patent makes use of a low level of binder. It also advocates the use of hydrophobic polymeric binders such as polyurethanes, cellulose acetates and poly(methylmethacrylate) in an element which has no gelatin layers on the backing layer side of the support. While this provides for very efficient vacuum drawdown, it has the disadvantage of poor opaquing performance since aqueous-based opaquing inks used in the graphic arts do not wet or adhere well to such hydrophobic backings. Moreover, such backing layers have only very limited water permeability and this severely inhibits the dissolution and removal of filter dyes during processing. The problem is an especially acute one in the field of graphic arts where very brief processing times are utilized to increase productivity.
(5) The backing layer must function to effectively resist aqueous alkaline developing solutions used in the processing of the photographic element. Thus, the backing layer must not dissolve in the developing solution or in other processing solutions. Moreover, the backing layer must be sufficiently hard that it resists softening and becoming tacky when the photographic element is processed. A soft and tacky surface is highly undesirable because of dirt pickup that occurs in processing machines. Thus, for example, foreign matter can be readily transferred from the rollers of the processing machine if the surface is soft and tacky. To avoid such problems, the backing layer should be applied and fully hardened during the manufacturing process.
In the manufacturing process, it is highly advantageous for the backing layer to be applied in a coating step that takes place prior to the coating of the silver halide emulsion layer. By this means, any waste, e.g., product that does not meet specification or has some unacceptable defect, associated with the coating application of the backing layer is at the cost of film support rather than at the cost of silver halide photographic emulsion coated product. This represents a significant cost saving. It is also important that the backing layer achieve its final properties quickly, without the need for any prolonged hardening or curing process. In this way, the physical properties of the backing layer can be tested before application of the costly silver halide emulsion layer and without any time delay or need for inventory control to allow for prolonged hardening or curing.
U.S. Pat. No. 4,977,065 describes a process for the production of a waterproof support material with an anticurl backing layer applied in two successive coating steps. The bottom layer comprises gelatin and a chromium (III) salt hardener, while the upper layer, that is applied after fully drying the bottom layer, comprises a chromium (III) salt-hardened non-gelatin layer. Glyoxal can also be added as an additional hardener. Reportedly, the process provides a curl control backing with extremely low water absorption. However, a backing layer with extremely low water absorption is not desirable for thorough dissolution of antihalation dyes nor for the application, drying, and adherence of water-based opaquing inks.
U.S. Pat. No. 5,077,185 describes a backing layer for graphic arts photographic elements that comprises a water-soluble electrically-conductive polymer having functionally attached carboxyl groups, a polyfunctional aziridine crosslinking agent, an antihalation dye and matte beads. Reportedly the layer provides antistatic and antihalation properties as well as low dye stain and improved dimensional stability. The layer was not reported to provide curl control.
European Patent Application No. 514,903, published Nov. 25, 1992, describes a backing layer that is reportedly useful for graphic arts photographic elements which comprises a bottom layer that is primarily gelatin and a top layer which comprises an acrylate polymer latex as the binder. However, for good opaquing and good dye dissolution, utilization of such a hydrophobic top layer is undesirable.
It is toward the objective of providing a novel photographic element, useful as an imaging master in the graphic arts that overcomes the disadvantages and limitations of the prior art, that this invention is directed. It is also an objective of the invention to provide a new and improved process for the manufacture of such photographic elements.